With the increasing awareness for sustainability and the environment, natural polymers have been the focus of much research. Much research has been directed at forming polymeric materials made of, or derived from, renewable biological materials.
Starch is an abundant, inexpensive, renewable natural polymer derived from plants. The glass transition temperature of pure dry starch is above its decomposition temperature, and therefore starch does not soften and flow, making it difficult to process. However, starch can be plasticized with relatively low levels (5-50 weight percent, preferably 10-40 weight percent, and most preferably from 10 to 30 weight percent) of plasticizers to produce thermoplastic starch (TPS) which will flow at elevated temperatures. Useful plasticizers are those that are capable of hydrogen bonding with the starch hydroxyl groups, such as water, and polyhydric alcohols like glycerol, ethylene glycol, mannitol and sorbitol.
While thermoplastic starch flows at high temperatures, it is a very brittle material, difficult to process, and the properties are poor in high humidity. The properties of TPS can be significantly improved by blending with other natural and synthetic polymers. Unfortunately, hydrophilic TPS is difficult to combine with hydrophobic polymers, such as polyolefins and other petroleum-derived polymers, leading a discontinuous, multi-stage morphology and poor mechanical properties. US 2012/0022188 describes blends of TPS with very low density polyolefin. The blend has a co-continuous phase morphology, and requires ethylene acrylic acid copolymer as a compatibilizer. Films of the blend were found to have excellent anti-static properties.
Blends of thermoplastic starch with thermoplastic elastomers has resulted in the formation of breathable films US 2012-0219781, FR 12.56142 and FR 12.56143. The formation and retention of static electricity charges at the surface of most plastics are known. For example, the presence of static electricity on thermoplastic films causes these films to stick to one another, making it difficult to separate them. The presence of static electricity on packaging films can cause the accumulation of dust on the objects to be packaged and thus impair their use. Static electricity can also damage microprocessors or components of electronic circuits. Static electricity can also cause the combustion or explosion of inflammable materials such as, for example, expandable polystyrene beads which contain pentane.
Antistatic agents for polymers are generally ionic surfactants of the ethoxylated amine or sulfonate type which are added to the polymers. However, the antistatic properties of the polymers incorporating these surfactants depend on the ambient humidity and they are not therefore permanent. This is because these surfactants have a tendency to migrate to the surface of the polymers and then to disappear.
Copolymers comprising polyamide blocks and hydrophilic blocks form antistatic agents which have the advantage of not migrating. Their antistatic properties are permanent and independent of the ambient humidity. U.S. Pat. Nos. 5,338,795, 5,965,206 and 6,825,270 all describe polymer substrates made antistatic by adding a copolymer comprising polyether blocks and polyamide blocks to their composition.
Thermoplastic elastomers (TPE) are used in the field of electronics alone and as additives, for their property of exceptional elastic springback. In applications of this type, the parts must be able to withstand both a high pressure and a high temperature so as not to risk being damaged, spoiled or deformed, nor to have their mechanical properties modified.
Thermoplastic elastomers, including block copolymers having polyether blocks and polyamide blocks such as PEBAX resins from Arkema, have good anti-static properties (surface resistivity of 109). US 2012/0108694 describes the improvement of the antistatic properties of PEBAX using organic salts (to 107). However, the use of salts can be detrimental to the use of the resin in food or medical applications.
It is desired to provide a thermoplastic plastic additive having improved anti-static properties. It has now surprisingly been found that alloys of thermoplastic elastomers and TPS can provide a 10 fold higher level of surface resistivity over the thermoplastic elastomer alone. Additionally, the TPS/TPE alloy is cost effective compared to the TPE alone, and increases the bio-based content of the final product. The anti-static additive is added at from 5 to 40 weight percent into a polymer matrix, providing both anti-static and mechanical enhancement.